An antibody that binds fibrin but not fibrinogen would be of great value in detecting thrombi in vivo. The requirements for specificity are dictated by the high concentration of fivroinogen in circulating plasma. A fibrin-specific antibody that did not react with fibrinogen when labelled with a radionuclide would permit imaging of fibrin deposited inthe vascular system. The strategy of this proposal relies on the synthesis of peptides that are unique to fibrin. These include peptides representing the end-terminus of the alpha chain and beta chain of fibrin, the gamma-gamma cross-linked peptide, and the alpha chain cross-linked acceptor peptide. These peptides will be conjugated to carrier proteins and used as immunogens. Rabbits will be immunized and fibrin-specific antibodies obtained from anti-sera by affinity chromatography utilizing fibrinogen-Sepharose, fibrin (monomer and cross-linked)-Sepharose, and peptide agarose supports. Mice will also be immunized with these peptide protein conjugates and somatic cell fusion effected in order to obtain hybridoma clones secreting monoclonal antibodies. Clones producing antibodies specific for fibrin will be amplified. After appropriate screening using in vitro immunoassay, the utility of fibrin-specific antibodies in vivo will be examined using a rabbit aorta in which endothelium is denuded according to the Baumgartner model and fibrin deposited. The need for these fibrin-specific antibodies lies in their potential as diagnostic agents of life-threatening or -debilitating thromboses.